Air/fuel control systems are known which are responsive to two-state (rich or lean) exhaust gas oxygen sensors positioned upstream of conventional three-way (HC, CO, NOx) catalytic converters. A feedback variable derived by integrating the upstream sensor output adjusts the engine air/fuel ratio in an attempt to maintain stoichiometric combustion. However, aging and manufacturing tolerances of the sensors may result in air/fuel operation at values other than stoichiometry.
In an effort to maintain the engine air/fuel ratio at stoichiometry, either the feedback variable or the upstream sensor output are biased in response to an output of another exhaust gas oxygen sensor positioned downstream of the converter. An example of such an approach is disclosed in U.S. Pat. No. 5,115,639.
The inventors herein have recognized numerous problems with the above approaches. For example, despite having a downstream exhaust gas oxygen sensor, the engine's air/fuel ratio may still not be maintained within the peak efficiency window of the catalytic converter. Such misalignment in operation may be caused by factors such as aging or variations in manufacturing tolerances of either the exhaust gas oxygen sensors or the catalytic converter.